1. Field of the Invention
The present invention relates to an optical fiber preform with a small transmission loss of light at wavelength of 1385 nm and small rise of the transmission loss caused by a hydroxy (OH) group in case of being exposed to hydrogen atmosphere, a method for manufacturing the optical fiber preform, and an optical fiber obtained by drawing the optical fiber preform.
2. Description of the Related Art
FIG. 1 shows an example of the configuration of a conventional optical fiber preform sintering apparatus 100. The sintering apparatus 100 has a container 14, a heater 22, a gas introduction pipe 24, and a drive source 16. The container 14 is made from silica glass. A heater 22 is arranged around the container 14 to heat the container 14.
The gas introduction pipe 24 is connected to the bottom part of the container 14, and the mixed gas, which contains inert gas such as helium (He) gas and dehydration-reaction-gas such as chlorine (Cl2) gas, is introduced into the container 14 through the gas introduction pipe 24.
An exhaust pipe 20 is connected to the top part of the container 14, and the mixed gas which travels through the container 14 from the bottom part of the container 14 is discharged from the exhaust pipe 20. The drive source 16 is provided in the upper part of the sintering apparatus 100. The drive source 16 is connected to a core rod 10.
The optical fiber preform 12 is formed around the circumference of the core rod 10 by such as VAD method before the dehydration process. The drive source 16 inserts the preform 12 into the container 14 by descending the core rod 10 into the container 14. The container 14 is filled with the atmosphere of the mixed gas, which flowed from the gas introduction pipe 24, and the circumference of the container 14 is heated by the heater 22. Therefore, the preform 12 inserted into the container 14 is heated under a mixed gas atmosphere to be dehydrated and sintered.
FIG. 2 shows relationship between a transmission loss and wavelength in a conventional general single mode optical fiber. Wavelength of light used in communication is mainly about 1300 nm or about 1550 nm because an inexpensive semiconductor laser can be used. As WDM (Wavelength Division Multiplexing) technology advances, light at wavelength band from 1300 nm to 1600 nm needs to be used in order to raise data-transmission capacity.
However, as shown in FIG. 2, the transmission loss in a general optical fiber rises sharply in wavelength of about 1385 nm. As the transmission loss becomes greater, the regenerator for amplifying and regenerating light needs to be added for a long distance transmission, which results in that cost of the whole transmission or communication system rises.
Accordingly, it is necessary to suppress an abrupt increase of the transmission loss at wavelength of about 1385 nm.
In addition, as shown in FIG. 2, a difference between a peak value of the transmission loss at wavelength of about 1385 nm and a value of the transmission loss in a case of decreasing gradually as shown by a broken line is defined as an OH peak hereinafter. For example, the OH peak shown in FIG. 2 is about 0.06 dB/km. The sharp rise or abrupt increase of the transmission loss at wavelength of about 1385 nm, i.e. the OH peak is caused by vibration of the OH group contained in the optical fiber and absorbing light of that wavelength. In order to decrease the OH group in the optical fiber, it is necessary to decrease the OH group in the preform which is a base material of the optical fiber.
Furthermore, even if the OH peak in the optical fiber just after drawing is sufficiently small, there is a possibility that the OH peak rises by hydrogen diffusing in the optical fiber, and reacting to a defect in a glass of the optical fiber, and then generating the OH group, if the optical fiber is exposed to hydrogen atmosphere for some reasons.
In FIG. 3, a dotted line shows a spectrum of the transmission loss in case that the optical fiber, the OH peak of which is sufficiently small as shown by a solid line, is exposed to atmosphere of 1% hydrogen for four days. FIG. 3 shows a rise of 0.1 dB/km of the OH peak at wavelength of about 1385 nm. The OH peak at wavelength of about 1240 nm is caused by hydrogen diffusing in the optical fiber. The OH peak disappears if the optical fiber is exposed to atmospheric air for a while and hydrogen is removed from the optical fiber. However, the rise of the OH peak at wavelength of about 1385 nm is irreversible and does not decrease. Therefore, the defect that causes the rise of the OH peak in the optical fiber needs to be reduced sufficiently.